Sorry but there is no free best anti virus. Be glad AVG wouldn't install on your machine. It sux. It's well known for letting Trojans walk right in. If you can't afford it right now you can still get the free 30 day trial of Kaspersky. I've been told that if you can delete all traces of it when the 30 days runs out then you can reload it again for another 30 days. But you have to get it all off of your machine before trying to reload it again. Kaspersky is the best on the market. You will not find any better. I've had close to a year. No infections of any kind.

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Avast is an excellent free antivirus. First I suggest you update to the newest version (4.8) and use the boot-time scan of 4.8 and quarantine anything it finds.

After that scan with Superantispyware free, Malwarebytes Antimalware free, F-secure Blacklight and the Kaspersky AVP Tool.

To keep your computer clean from malware, I suggest you stick with Avast. Occasionally scan with Superantispyware free and Malwarebytes Antimalware free, you can also scan with Spybot (and turn on the Immunization function).

For browsing, you should use Firefox (with the Adblock Plus addon) or Opera instead of Internet Explorer or Safari - this reduces the chance of malware getting on your computer.